Doug Mahugh of Microsoft is pushing inside the ODF Technical Committee for proprietary extensions, by which the monopolist vendor could embrace and extend the format to "innovate". The extensions possibility is the door open to proprietary closed source parts, that renders the ODF customer a Microsoft slave once again like in the good old times of the .DOC.
Here is what Doug Mahugh is saying on the ODF mailing-list:
It's worth noting that the ODF metadata mechanisms don't allow for the use of a private/custom schema to tag content within a document. And that use case has value to many users. So if we decide that ODF won't be able to support those types of scenarios, for whatever reason, we should not be surprised to find that users who need such capabilities will look elsewhere.
Consider the trivial example of a pre-existing document, created years ago, which needs to be logged in to a content management system that requires an abstract to be identified for each document. If the format of the document is HTML, then a div with class="abstract" can be used to tag the appropriate paragraph(s) as the abstract. If the format of the document is DOCX, a customXml element with element="abstract" can be used for the same purposes. In both cases the document content remains valid HTML or WordprocessingML, while the user adds the custom semantics required for their purpose. The custom semantics can be (and should be) ignored by others. The user is free to innovate quickly, and does not have to think in terms of a tradeoff between strict compliance and flexibility/business value. They can, and do, have the best of both worlds in such scenarios: strict compliance to a standard, and freedom to innovate quickly for their own specialized purposes.
I think ODF would benefit from being as supportive of such scenarios as HTML, IS29500 and other formats already are. No committee can anticipate every possible class of extension that users might find useful, so I think the format itself should allow for clean, simple tagging of content according to schemas that may never be standardized, and may never be widely known or used. Done correctly, such tagging puts no burden on simple interoperability between word processors (which typically ignore it), but can enable other types of interoperability that many people find valuable.
There is only one way to do interoperability at Microsoft: embrace and extend. "Freedom to innovate" here means "Freedom to embrace and extend", or "Freedom for Microsoft to add proprietary patented extensions that makes users dependent on Microsoft technologies".
Time to join the ODF TC!